This invention relates generally to the recovery of carbon dioxide and, more particularly, to the recovery of carbon dioxide from a feed mixture which also contains oxygen.
Carbon dioxide has a large number of uses. For example, carbon dioxide is used to carbonate beverages, to chill, freeze and package seafood, meat, poultry, baked goods, fruits and vegetables, and to extend the shelf-life of dairy products. It is an important environmental component in industrial waste and process water treatment as a replacement for sulfuric acid to control pH levels. Other uses include drinking water treatment, an environmentally friendly pesticide and an atmosphere additive in greenhouses to improve the growth of vegetables.
Generally carbon dioxide is produced by purifying a waste stream which is a by-product of an organic or inorganic chemical process. The waste stream, which comprises a high concentration of carbon dioxide, is condensed and purified in multiple stages and then distilled to produce the product grade carbon dioxide.
As the demand for carbon dioxide continues to increase, alternate sources of carbon dioxide are being used to supply the crude carbon dioxide feed to the purification system. Such alternate feeds have a much lower concentration of carbon dioxide and thus need to be upgraded, i.e. the concentration of the carbon dioxide must be increased, before product grade carbon dioxide can be effectively produced. These alternate feeds with much lower carbon dioxide concentrations are referred to as lean feeds. An example of such a lean feed is flue gas from, for example, a combustion source such as a boiler, internal combustion engine, gas turbine or lime kiln.
Upgrading of the carbon dioxide concentration in a feed can be carried out in a number of ways. One particularly preferred method is the chemical absorption of carbon dioxide from the crude carbon dioxide feed into an alkanolamine based absorbent. The resulting carbon dioxide loaded absorbent then undergoes separation into carbon dioxide product for recovery and into alkanolamine containing absorbent which is may be recycled for reuse within the recovery system.
Often the crude carbon dioxide feed contains significant levels of oxygen which can cause degradation of the alkanolamines reducing their utility in the recovery system and also causing corrosion problems in the system. Those skilled in the art have addressed this problem in one of two ways. In one method, chemical inhibitors are added to the absorber fluid to protect against degradation by inhibiting the oxidation of the alkanolamines. In another method, the oxygen is removed from the crude carbon dioxide feed prior to the interaction of the crude carbon dioxide feed with the alkanolamine based absorbent. In one example of this method, a combustible fuel is added to the crude carbon dioxide feed for combustion with the oxygen in a catalytic combustion reaction. While both methods are effective they are both characterized by high capital costs and, moreover, are complicated to operate.
Accordingly, it is an object of this invention to provide a system which can more effectively recover carbon dioxide or other absorbate from an oxygen containing feed using an alkanolamine based absorbent to upgrade the feed.
The above and other objects, which will become apparent to one skilled in the art upon a reading of this disclosure, are attained by the present invention which is:
Apparatus for recovering absorbate from an oxygen-containing feed mixture comprising:
(A) an absorption column, means for passing a feed mixture comprising oxygen and absorbate into the lower portion of the absorption column, and means for passing absorbent comprising at least one alkanolamine into the upper portion of the absorption column;
(B) an oxygen separator and means for passing fluid from the lower portion of the absorption column into the oxygen separator;
(C) a heat exchanger and means for passing fluid from the oxygen separator to the heat exchanger;
(D) a stripping column and means for passing fluid from the heat exchanger to the upper portion of the stripping column; and
(E) means for recovering absorbate from the upper portion of the stripping column.
As used herein, the term xe2x80x9cabsorption columnxe2x80x9d means a mass transfer device that enables a suitable solvent, i.e. absorbent, to selectively absorb the absorbate from a fluid containing one or more other components.
As used herein, the term xe2x80x9cstripping columnxe2x80x9d means a mass transfer device wherein a component such as absorbate is separated from absorbent, generally through the application of energy.
As used herein, the term xe2x80x9cflash tankxe2x80x9d means a vessel that allows for phase separation of a dissolved gas from a liquid through pressure reduction such as by the operation of a vacuum pump.
As used herein, the term xe2x80x9cinhibitorxe2x80x9d means a chemical or mixture of chemicals that inhibits or reduces the rate of a reaction. For example, copper carbonate in combination with one or more of dihydroxyethylglycine, alkali metal permanganate, alkali metal thiocyanate, nickel or bismuth oxides with or without alkali metal carbonate inhibits oxidative degradation of an alkanolamine.
As used herein the term xe2x80x9coxygen scavenging gasxe2x80x9d means a gas that has an oxygen concentration less than 2 mole percent, preferably less than 0.5 mole percent, and which can be used to strip dissolved oxygen from a liquid.
As used herein, the terms xe2x80x9cupper portionxe2x80x9d and xe2x80x9clower portionxe2x80x9d mean those sections of a column respectively above and below the mid point of the column.
As used herein, the term xe2x80x9cindirect heat exchangexe2x80x9d means the bringing of two fluids into heat exchange relation without any physical contact or intermixing of the fluids with each other.